Geth is the most popular client for running an Ethereum node. It is written in the Go programming language and is the main software used by the Ethereum network.
Geth is also used to develop and test smart contracts on the Ethereum blockchain.
Geth implements the Ethereum Virtual Machine (EVM), which is responsible for executing smart contracts on the Ethereum network. The EVM is a Turing-complete virtual machine that runs on top of a blockchain.
Turing-complete means that the EVM can run any program that can be run on a computer.
Geth also includes a built-in debugger for debugging smart contracts. The debugger can be used to step through the execution of a smart contract and see the values of variables at each step.
Geth is open source software and is available for download from the Ethereum Foundation website.